1. Field of the Invention
The subject invention relates to tunneling susceptometry, which includes determination of magnetic susceptibility or, broadly, of susceptibility of a material to a field, with the aid of electric tunneling current, and also relates to methods and apparatus for enabling or effecting such susceptibility determinations or investigations.
2. Information Disclosure Statement
The following disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior art, inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material though not necessarily being of itself pertinent. Also, the following comments contain conclusions and observations which have only been drawn or become apparent after conception of the subject invention or which contrast the subject invention or its merits against the background of developments which may be subsequent in time or priority.
The subject invention should be distinguished from Scanning Tunneling Microscopy (STM), even though embodiments thereof may in part use similar instrumentation. STM sprang from efforts to characterize the topography of surfaces at the atomic level, manifesting themselves initially in the so-called "topografiner" developed by Russell Young, John Ward and Fredric Scire, as apparent from their article entitled The Topografiner: An Instrument for Measuring Surface Microtopography, Rev. Sci. Instrum., 1972, 43, 999. The topografiner produced real space images of irregular surfaces. Since the topografiner achieved lateral resolutions on the order of 4000 .ANG. and surface normal resolutions of 30-40 .ANG., it was a notable development of the past six years when Binnig et al. overcame various stability problems and demonstrated the first scanning tunneling microscope which achieved lateral resolutions on the order of tens of angstroms, with angstrom resolution normal to the surface. Early STM designs went to great lengths to achieve tunneling gap stability. As apparent from their article entitled Tunneling Through A Controllable Vacuum Gap, G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Appl. Phys. Lett., 1982, 40, 178, demonstrated tunneling from a W tip to a Pt surface with an STM in a vacuum chamber on a stone bench "floating" on inflated rubber tubes. Internal vibrations were filtered out by magnetic levitation over a superconducting bowl of Pb which was superinsulated and cooled directly by liquid He. Subsequently, tunneling current has been demonstrated with less elaborate STM units. The key is structural rigidity of the tip-sample connection which forces any external vibrations to move tip and sample identically and simultaneously. There now are STM's with lateral resolutions of less than 5 .ANG. and normal resolutions of hundredths of angstroms for certain surfaces.
In an article entitled Atomic Force Microscope, Phys. Rev. Lett., 1986, 56, 930, Drs. G. Binnig, C. F. Quate and Ch. Gerber proposed measurement of ultrasmall forces on particles as small as single atoms by monitoring the elastic deformation of various types of springs with their scanning tunneling microscope. By way of background, they pointed out that it has been a common practice to use the displacement of springs as a measure of force, and that previous methods have relied on electrostatic fields, magnetostatic fields, optical waves, and x-rays. They also commented that SQUIDs are superconducting elements that measure the expulsion of magnetic fields in variable-inductance devices, and that have been used in gravity gradiometers to measure displacements of 10.sup.-6 .ANG.. Others in their work with van der Waals forces have used optical interference methods to measure displacements of 1 .ANG.. Their effort similarly was concerned with penetrating the regime of interatomic forces between single atoms and they proposed their atomic force microscope (AFM) as a new tool designed to exploit that level of sensitivity, enabling surface investigation of both conductors and insulators on an atomic scale. They envisioned a general-purpose device that will measure any type of force; not only the interatomic forces, but electromagnetic forces as well. However, the atomic force microscope actually disclosed in that article is a combination of the principles of the scanning tunneling microscope and the stylus profilometer.
In their proposed first mode, they modulated the sample in the z-direction at its resonant frequency (5.8 kHz); that is, in the direction of tunneling current flow. The force between the sample and the diamond stylus-the small force that they want to measure-deflects the lever holding the stylus. In turn, this modulates the tunneling current which is used to control the AFM-feedback circuit and maintain the force f.sub.o at a constant level.
In their second and third modes, the lever carrying the diamond stylus is driven at its resonant frequency in the z-direction with an amplitude of 0.1 to 10 .ANG.. The force, f.sub.o, between sample and stylus changes the resonant frequency of the lever. This changes both the amplitude and phase of the ac modulation of the tunneling current. Either of these can be used as a signal to drive the feedback circuits.
In the fourth mode they used one feedback circuit. It was connected to the AFM and it was controlled by the tunneling current in the STM. This system maintained the tunneling gap at a constant level by changing the force on the stylus.
The fourth mode was further improved by reconnection of both feedback circuits in such a way that the AFM sample and the STM tip were driven in opposite directions with a factor .alpha. less in amplitude for the STM tip. The value of .alpha. ranged from 10 to 1000.
In contrast to previous methods, the absolute value of f.sub.o, the force on the stylus, was not well defined except at the beginning of the measurement, even in the absence of thermal drifts. However, they saw the limiting sensitivity of their instrument as far less than interatomic forces ranging from ionic bonds to van der Waals bonds and down to perhaps 10.sup.-12 N for some of the weaker forces of surface reconstruction. Their AFM, therefore, should be able to measure all of the important forces that exist between the sample and ad atoms on the stylus.
They further pointed out that these forces also exist in the tunneling microscope itself and that they can have a strong influence on the data collected with the STM. Accordingly, they mentioned that the STM could be used as a force microscope in the mode they described by simply mounting the STM tip on a cantilever beam.
Further background materials include another article by G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, of the IBM Zurich Research Laboratory, entitled Surface Studies By Scanning Tunneling Microscopy, Phys. Rev. Lett., 1982, 49, 57, an article by U. Durig, J. K. Gimzewski and D. W. Phol, entitled Experimental Observation of Forces Acting During Scanning Tunneling Microscopy, Phys. Rev. Lett., 1986, 57, 2403, and another article on atomic force microscopy by Gary M. McClelland, Ragnar Erlandsson and Shirley Chiang, entitled Atomic Force Microscopy: General Principles and a New Implementation, accepted for publication in Review of Progress in Quantitative Non-Destructive Evaluation, vol. 6, Plenum, New York (1987).
An intersecting design is apparent from an article by Ch. Gerber, G. Binnig, H. Fuchs, O. Marti and H. Rohrer, entitled Scanning Tunneling Microscope Combined with a Scanning Electron Microscope, Rev. Sci. Instrum., 1986, 57, 221, disclosing their "Pocket-Size" STM needing very little external vibration isolation.
Scanning tunneling microscopy also has been described in an article thus entitled by G. Binnig and H. Rohrer, published in Helvetica Physica Acta, vol. 55 (1982) 726-735.
In 1986, Drs. Binnig and Rohrer received the Nobel Prize in physics for their above mentioned work.
An overview has been published by Calvin F. Quate under the title of Vacuum tunneling: A new technique for microscopy, Phys. Today (Aug. 1986) 26-33, mentioning inter alia topography, surface state studies, surface charge density measurements, catalytic reaction studies, and material deposition as present or prospective fields of application.
A so-called "tube scanner" has been disclosed by G. Binnig and D. P. E. Smith in an article entitled Single-tube Three-dimensional Scanner for Scanning Tunneling Microscopy, Rev. Sci. Instrum., Aug. 1986, 57, 1688.
An advanced scanning tunneling microscope has been disclosed by W. J. Kaiser and R. C. Jaklevic, in Scanning Tunneling Microscopy Study of Metals: Spectroscopy and Topography, Surf. Sci. 181, 55 (1987). That Scanning Tunneling Microscope system was operated in several environments for both topographic imaging and tunnel spectroscopy. It shows high resistance to the effects of vibration and thermal drift. The device is unique in its simplicity and has only four moving parts. In addition, the critical tip-sample approach mechanism is inherently reliable and precise. That STM system accommodates a wide range of sample geometries and requires no special sample holder.
In a different vein, measurement of magnetic susceptibility has become increasingly important. Reference may in this respect be had to an article by Robert E. Benfield, Peter P. Edwards and Angelica M. Stacy, entitled Paramagnetism in High-nuclearity Osmium Clusters, J. Chem. Soc., Chem. Commun., 1982, 10, 525, mentioning a Faraday apparatus for measurement of magnetic susceptibility, and to an article by B. J. Pronk, H. B. Brom, L. J. de Jongh, G. Longoni and A. Ceriotti, entitled Physical Properties of Metal Cluster Compounds I: Magnetic Measurements on High-Nuclearity Nickel and Platinum Carbonyl Clusters, Solid State Commun., 1986, 59, 349, mentioning use of either a sensitive pendulum magnetometer or a vibrating sample magnetometer with a 6 T superconductive magnet for magnetic susceptibility measurement.
Changes in magnetic susceptibility with changing magnetic field strength have also been measured, such as by means of the de Haas - van Alphen effect exploring the Fermi surface of a sample, as mentioned by R. G. Chambers, in Canadian J. Phys., 1956, 34, 1395, at 1407 et seq.
A magnetic susceptometer for thin films and surfaces capable of detecting the effect of 10.sup.-6 atomic layers of Fe on a superconducting Pb substrate has been described by R. Maservey and J. S. Moodera, in an article entitled Performance of a Magnetic Susceptometer for Thin Films and Surfaces, J. Appl. Phys., 1986, 60, 3007. That method relies on measurements of the change in inductance of a 1000 .ANG. thick superconducting Nb meander line. The sample to be measured is evaporated onto the insulator encapsulating the Nb meander line, which is the inductive element of a resonant circuit including a capacitor and being driven by a tunnel diode circuit. Changes in resonant frequency are indicated by a counter and plotted as a function of film thickness as measured by a quartz thickness gauge.
A clear advantage of that system is its ability to measure surface adsorbate susceptibilities with extreme sensitivity. However, the system can only be operated at superconducting temperatures. The geometry is restricted, in that orientation effects are at least difficult to study. Substrate choice is limited to substances which can be evaporated on the insulator surface, at least in the setup disclosed in that article.
A Superconducting Quantum Interference Device (SQUID) has been described by S. Vitale, S. Morante and M. Cerdonio, in an article entitled Superconducting Susceptometer for High-Accuracy Routine Operation, Rev. Sci. Instrum., 1982, 53, 1123.